Water-Soluble Nitroxyl Porphyrin Complexes FeIITPPSHNO and FeIITPPSNO- Obtained from Isolated FeIITPPSNO•.

The first biomimetic water-soluble FeII-porphyrin nitroxyl complexes were obtained and characterized by UV-vis in protonated and deprotonated forms by reduction of previously isolated and characterized FeIITPPSNO•. The pKa involved in the FeII-HNO ⇄ FeII-NO- + H+ equilibrium was estimated to be around 9.7. The FeIITPPSHNO complex spontaneously reoxidizes to the nitrosyl form following a first-order kinetic decay with a measured kinetic constant of k = 0.017 s-1. Experiments show that the HNO adduct undergoes unimolecular homolytic cleavage of the H-NO bond. DFT calculations suggest a phlorin radical intermediate for this reaction. The deprotonated NO- complex resulted to be more stable, with a half-life of about 10 min.